We
report a rare atom-like interaction between excitons in monolayer
WS<sub>2</sub>, measured using ultrafast absorption spectroscopy.
At increasing excitation density, the exciton resonance energy exhibits
a pronounced redshift followed by an anomalous blueshift. Using both
material-realistic computation and phenomenological modeling, we attribute
this observation to plasma effects and an attraction–repulsion
crossover of the exciton–exciton interaction that mimics the
Lennard-Jones potential between atoms. Our experiment demonstrates
a strong analogy between excitons and atoms with respect to interparticle
interaction, which holds promise to pursue the predicted liquid and
crystalline phases of excitons in two-dimensional materials.